1. Field of the Invention
This invention relates to circuit protection devices that limit or shut off current flow in conditions of over-current and/or over-temperature.
2. Description Of Related Technology
Raychem Corporation, Menlo Park, California markets a circuit protection device called a "polyswitch." Raychem's "polyswitch" includes a polymeric material loaded with conductive material such as carbon particles which is normally conductive. If the current load increases beyond a predetermined value, the polymer heats up and expands with the result that the conductive particles are separated enough to prevent flow of current through the polymer. A problem with this polymeric type device is that it has an undesirably slow response time due to low thermal conductivity of the polymeric materials. Accordingly, there is a need in the art for a device which quickly changes from a low resistance to a high resistance when an over-current or an over-temperature condition exists.
Another problem with the polymeric device is internal "arcing" which occurs when the current flow is interrupted between adjacent particles. This internal "arcing" leads to breakdown of the polymer and hence limits the upper voltage which can be applied to the device. Accordingly, there is a need in the art for a more reliable switch capable of performing under higher voltage and current conditions.
Another inherent problem of polymeric devices is that the conductivity is relatively low even in its most conductive state. As a result, high current devices are undesirably large in size when low resistance levels are required.
Ceramic PTC (positive temperature coefficient) devices based on barium titanate perform very similarly to polymeric devices and also display catastrophic breakdown when exposed to elevated voltage and/or current conditions.
Various types of mechanical switching arrangements are known in the art. For instance, U.S. Pat. No. 3,544,943 ("Hoagland") discloses an over-current responsive device which includes a pair of terminals electrically connected together by a thermally responsive element. The thermally responsive element includes two elongated cantilevered members supported at one end to a pair of posts. The posts are electrically connected to the terminals. The first elongated member is electrically insulated from the posts. One end of the second elongated member is welded to a free end of the first member. The second member is also bifurcated into two arms, one arm being electrically connected to one post and the other arm being electrically connected to the other post. Current flows from one terminal, along one arm, then along the other arm to the other terminal. The size, shape, and/or materials of the first and second members are chosen such that the second member is heated and the two members swing in one direction to activate a snap-action switch under overload conditions.
Shape memory alloys have been used in electrical connectors. For instance, U.S. Pat. No. 4,621,882 ("Krumme") discloses an electrical connector wherein a first strip which terminates in a split tube is removably connected to a second strip. The split tube includes a shape memory alloy layer which opens or closes the tube. For instance, the tube can include a metal layer which acts as a spring to close the tube when the shape memory layer is in its ductile and soft martensitic state and the shape memory layer changes shape and overpowers the force of the metal layer when the shape memory layer is heated to its austenitic state. The tube can include a flexible heater for heating the shape memory layer.
U.S. Pat. No. 4,643,500 ("Krumme") discloses a multi-contact zero insertion force electrical connector. In a first embodiment, the connector includes a pair of flexible spaced-apart sidewalls, slides having camming surfaces extend along inner surfaces of the sidewalls, pairs of spaced-apart contacts are provided between the sidewalls, upper ends of the contacts are attached to the respective sidewalls by extensions on the sidewalls, and the slides are pushed and pulled by means of a shape memory U-shaped Nitinol (nickel-titanium) wire which extends around the sidewalls with free ends of the wire connected to terminals. To insert a printed circuit board between the sidewalls, current is applied across the terminals to heat the wire to its austenitic state which causes the wire to shrink to a memory state. As a result, the upper portions of the sidewalls are pushed apart by the slides. Upon cooling of the wire, the sidewalls move towards each other and the contacts clamp the circuit board in place.
In another embodiment, Krumme discloses opposed pairs of contacts supported in a body, a U-shaped bail is slidably supported between the contacts, an S-shaped Nitinol member is between the body and the bail, and a pair of leads are connected to the Nitinol member for heating thereof or heating a heater bonded thereto. When the Nitinol member is heated to its austenitic state it expands and pushes up on the bail which in turn pushes the contacts apart. The Nitinol member can be covered with insulation to prevent electrical contact with the contacts.
U.S. Pat. No. 4,734,047 ("Krumme") discloses a multi-contact zero insertion force electrical connector. In a heat-to-open embodiment, a plurality o fork-shaped contacts include distal ends for holding a substrate. A split tube of a shape memory alloy is provided between the distal ends for spreading the distal ends when the alloy is heated to its austenitic state. A spring is concentrically layered with respect to the tube for deforming the tube when the alloy is in its martensitic state. The alloy is heated by a heater located within the tube. Alternatively, in a cool-to-open embodiment, the spring can be provided within the tube and the contacts are opened by cooling the alloy to its martensitic state whereby the spring expands the tube to spread the distal ends. The spring can be eliminated in the heat-to-open embodiment since the contacts are resilient and will deform the tube when the alloy is in its martensitic state. In addition, the tube can be resistance heated by passing a current therethrough.
U.S. Pat. No. 4,881,908 ("Perry") discloses a connector having a spring in the form of an elongated split tube and a heat-recoverable member of shape memory alloy positioned within the tube. Opposed sets of contact pads are positioned between the ends of the spring and are movable into and out of contact with a substrate inserted between the contact pads. To open the connector, the shape memory alloy is heated by passing a current therethrough or by using a resistance heater circuit or a separate resistance heater. For instance, a heater can be provided between the spring and the shape memory alloy. When the shape memory alloy is in a deformable state below a transition temperature, the spring deforms the shape memory alloy to close the connector. When the shape memory alloy is in a memory state above the transition temperature, the shape memory alloy recovers to its non-deformed state.